The present invention is generally related to an inkjet printer printhead and is more particularly related to a printhead orifice plate and the method for producing same in which multiple related orifices may be easily formed in the orifice plate.
An inkjet printer forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land in desired locations on the medium. In its simplest form, such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet printer comprises an ink containment section, which stores and supplies ink as needed, and a printhead, which heats and expels the ink droplets as directed by the printer control software. Typically, the printhead is a laminate structure including a semiconductor base, a barrier material structure which is honeycombed with ink flow channels, and an orifice plate which is perforated with small holes or orifices arranged in a pattern which allows ink droplets to be expelled.
In one conventional variety of inkjet printer the expulsion mechanism consists of a plurality of heater resistors formed in the semiconductor substrate which are each associated with one of a plurality of ink firing chambers formed in the barrier layer and one orifice of a plurality of orifices in the orifice plate. Each of the heater resistors is connected to the controlling software of the printer such that each of the resistors may be independently energized to quickly vaporize a portion of ink into a bubble which subsequently expels a droplet of ink from an orifice. Ink flows into the firing chamber formed in the barrier layer around each heater resistor and awaits energization of the heater resistor. Following ejection of the ink droplet and collapse of the ink bubble, ink refills the firing chamber to the point where a meniscus is formed across the orifice. The form and constriction in barrier layer channels through which ink flows to refill the firing chamber establish both the speed at which ink refills the firing chamber and the dynamics of the ink meniscus. Further details of printer, print cartridge, and printhead construction may be found in the Hewlett-Packard Journal, Vol. 36, No. 5, May 1985, and in the Hewlett-Packard Journal, Vol. 45, No. 1, February 1994.
One of the problems faced by designers of print cartridges is that of maintaining a high print quality while achieving a high rate of printing speed. When a droplet is expelled from an orifice due to the rapid boiling of the ink inside the firing chamber, most of the mass of the ejected ink is concentrated in the droplet which is directed toward the medium. However, a small portion of the expelled ink resides in a tail extending from the droplet to the surface opening of the orifice. This tail and associated ink spray can land on the medium resulting in a hazy printed image or character. The spray problem has been addressed by reducing the speed of the printing operation or by optimizing the architecture or geometry of the ink firing chamber and the associated ink feed conduits in the barrier layer. Orifice geometries also affect spray, see U.S. patent application Ser. No. 08/608,923, "Asymmetric Printhead Orifice" filed on behalf of Weber et al. on Feb. 29, 1996.
One conventional method of fabricating an orifice plate utilizes an electroless plating technique on a prefabricated mandrel. Such a mandrel is illustrated (not to scale) in FIG. 1, in which a substrate 101 has at least one flat surface constructed of silicon or glass. Disposed on the flat surface of the substrate 101 is a conducting layer 103, generally a film of chromium or stainless steel. A vacuum deposition process, such as the planar magnetron process, may be used to deposit this conductive film 103. Another vacuum deposition process may be used to deposit a dielectric layer 105, which typically is silicon nitride, and is deposed by a vacuum deposition process such as a plasma enhanced chemical vapor deposition process. Dielectric layer 105 is desirably very thin, typically having a thickness of approximately 0.30 um. Dielectric layer 105 is masked with a photoresist mask, exposed to UV light, and introduced into a plasma etching process which removes most of the dielectric layer except for "buttons" of dielectric material in preselected positions on the conductive layer 103. Of course, these positions are predetermined to be the location of each orifice of the orifice plate which is to be created atop the mandrel.
This reusable mandrel is placed into an electroforming bath in which the conducting layer 103 is established as a cathode while a base material, typically nickel, is established as the anode. During the electroforming process, nickel metal is transferred from the anode to the cathode and the nickel (shown as layer 107) attaches to the conductive areas of the conductive layer 103. Since the nickel metal plates uniformly from each conductive plate of the mandrel, once the surface of the dielectric button 105 is reached, the nickel overplates the dielectric layer in a uniform and predictable pattern. The parameters of the plating process, including the time of plating, are carefully controlled so that the opening of the nickel layer 107 formed over the dielectric layer button 105 is a predetermined diameter (typically about 45 um) at the dielectric surface. This diameter is usually one third to one fifth the diameter of the dielectric layer button 105 thereby resulting in the top layer of the nickel 107 having an opening at the inside surface 115 of the orifice plate of diameter d2 which is approximately three to five time the diameter of d1 of the opening which will be the orifice aperture at the outside surface 213 of the orifice plate. At the completion of the electroless plating process, the newly formed orifice plate is removed from the mandrel and gold plated for corrosion resistance of the orifice. Additional description of metal orifice plate fabrication may be found in U.S. Pat. Nos. 4,733,971; 5,167,766; 5,443,713; and 5,560,837, each assigned to the assignee of the present invention.
In ink-jet technology, which uses dot matrix manipulation to form both images and alphanumeric characters, the colors and tone of a printed image are modulated by the presence or absence of drops of ink deposited on the print medium at each target picture element (known as "pixels") of an imaginary superimposed rectangular grid overlay of the image. The luminance continuity-tonal transitions within the recorded image-is especially affected by the inherent quantization effects of using ink droplets and dot matrix imaging. The imaging system can also introduce random or systematic luminance fluctuations, commonly known as graininess-the visual recognition of individual dots with the naked eye. It has been estimated that the unaided human visual system will perceive individual dots until they have been reduced to less than or equal to approximately twenty to twenty-five microns in diameter in the printed image. Therefore, undesirable quantization effects of the dot matrix printing method are reduced in the current state of the art by decreasing the size of each drop and printing at a high resolution. Generally, a 1200 dots per inch ("dpi") printed image looks better to the eye than a 600 dpi image which in turn improves upon 300 dpi, etc. Additionally, undesired quantization effect can be reduced by utilizing more pen colors with varying densities of color (e.g., two cyan ink print cartridges, each containing a different ratio of dye to solvent in the chemical composition of the ink) or containing different types of chemical colorants.
One apparatus for improving print quality is discussed in an article, Bubble Ink-Jet Technology with Improved Performance, by Enrico Manini, Olivetti, presented at IS&T's Tenth International Congress on Advances in Non-impact Printing Technologies, Oct. 30-Nov. 4, 1994, New Orleans, La. Manini shows a concept for, "better distributing the ink on the paper, by using more, smaller droplets . . . utiliz[ing] several nozzles for each pressure chamber, so that a fine shower of ink is deposited on the paper." Sketches are provided by Manini showing two-nozzle pressure chambers, three-nozzle chambers, and four-nozzle chambers. Additional improvements to multi-nozzle (or multi-orifice) technologies are disclosed in U.S. patent application Ser. No. 08/812,385, "Method and Apparatus for Improved Ink-Drop Distribution in Ink-Jet Printing", filed on behalf of Weber et al. on Mar. 5, 1997 and assigned to the assignee of the present invention.
While the advantages of multiple orifices for printing are becoming clear, the techniques for realizing printheads with closely-spaced printing orifices particularly clusters of such orifices-are not well developed. The conventional process of manufacturing multiple orifices employs multiple non-conductive buttons on a conductive substrate mandrel, upon which a nickel or other metal is electrodeposited. Unfortunately, this method does not work well for clustered printing orifices because the limited space between heater resistors and associated firing chambers does not allow individual buttons to form orifices while allowing the orifice plate to be sufficiently thick to meet the architecture design requirements for print performance and the manufacturing requirements of orifice sheet strength and ease of handling.